Temperature dependence of intraparticle diffusion of coumarin in single octadecylsilyl-functionalized silica particle revealed by fluorescence microspectroscopy

Abstract We explore the intraparticle diffusion of coumarin 102 (C102) in an octadecylsilyl-functionalized (ODS) silica particle in acetonitrile (ACN)/water and butanol (BuOH)/water systems at varying temperatures (T). The enthalpy-entropy compensation plots of the distribution coefficient of C102 in the ODS silica particle found that the distribution mechanism differed between ACN and BuOH. The T-dependent intraparticle diffusion coefficient (Dintra) of C102 in a single particle was determined with fluorescence microscpectroscopy using laser trapping and fluorescence recovery after photobleaching. The obtained Dintra was analyzed based on Arrhenius plots and the pore and surface diffusion model. As a result, we revealed that C102 transferred in the ODS silica particle in the ACN/water solution through only surface diffusion, while both pore and surface diffusions were observed in the BuOH/water solution. It was found that the formation of an interfacial liquid phase was the reason for the observed surface-only diffusion in the ACN system. This was supported by the activation energy of Dintra and surface diffusion coefficient. Moreover, we demonstrate for the first time that the proposed system allows the evaluation of T-dependence of Dintra in a single particle.